Groups III-V compound semiconductors, e.g., GaAs, Al.sub.x Ga.sub.1-x As, GaAs.sub.x P.sub.1-x, In.sub.x Ga.sub.1-x As, and In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y, are extremely useful as materials for Gunn diodes, ultra-high speed semiconductor devices, light-emitting devices, etc., and the demand therefor has recently been considerably increasing. Compound semiconductor epitaxial crystals for these devices are produced by vapor-phase growth, molecular beam epitaxial growth, and liquid-phase growth. In particular, the vapor-phase growth method is attracting attention as an industrial method applicable to mass production.
Known techniques of vapor-phase growth of Groups IIIV compound semiconductors include (1) a metalorganic chemical vapor deposition (hereinafter referred to as MOCVD) method in which an alkyl compound of the Group III element and a hydride or alkyl compound of the Group V element are heat decomposed, (2) a hydride method using a chloride of the Group III element and a hydride of the Group V element, and (3) a chloride method using a chloride of the Group III element and a chloride of the Group V element. In the production of compound semiconductors containing arsenic by the MOCVD method and hydride method, arsine is widely used as a source of arsenic. By combining arsine with an alkyl compound of the Group III element in the MOCVD method or with a chloride of the Group III element in the hydride method, it is possible to grow a crystal exhibiting satisfactory light emission characteristics and, hence, industrial production of light-emitting devices, such as semiconductor lasers and light-emitting diodes has already been put into practice.
In particular, GaAs and Al.sub.x Ga.sub.1-x As (wherein 0&lt;X&lt;1) compound semiconductors are of extreme use as materials of ultra-high speed FET devices, and the demand therefor has recently been increasing in the field of various amplifiers and high-speed integrated circuits. FET devices for these applications are generally produced by processing a GaAs or Al.sub.x Ga.sub.1-x As crystal layer formed on a semi-insulating single crystal substrate through epitaxial growth so as to have prescribed carrier concentration, thickness and composition. The epitaxial crystal to be used in the FET device is produced by vapor-phase growth, molecular beam epitaxial growth or liquid-phase growth. In particular, the MOCVD method using an organic metal and arsine as raw materials has been attracting attention as an industrially applicable mass-production method. In the production of an epitaxial crystal for, for instance, a high electron mobility transistor (hereinafter abbreviated as HEMT) which has been recently noted as one of ultra-high speed FET according to the MOCVD method, arsine, trimethylgallium, trimethylaluminum, and a dopant gas are successively supplied onto a heated substrate of a GaAs single crystal and heat decomposed to form a non-doped GaAs layer (about 0.5 .mu.m thick), a non-doped Al.sub.0.3 Ga.sub.0.7 As layer (0.001 to 0.02 .mu.m thick), an N-type Al.sub.0.3 Ga.sub.0.7 As layer (0.03 to 0.05 .mu.m thick), and an N-type GaAs crystal (0.05 to 0.15 .mu.m thick) through epitaxial growth in a successive manner to thereby obtain a crystal having a prescribed structure. Epitaxial crystals applicable to the other FET devices can be prepared in a similar manner. Since the crystal thickness and composition are easily and precisely controllable by adjusting flow rates of raw material gases, the MOCVD method is expected as an advantageous technique of crystal growth for FET.
However, the conventional vapor-phase growth method using arsine as a Group V source has poor reproducibility in the formation of epitaxial crystals for use particularly in high-speed electronic devices requiring a high purity layer with a low impurity concentration. For example, in an FET used as an amplifier in the ultra-high frequency band, an N-type GaAs crystal active layer having an electron concentration of from about 1 to 2.times.10.sup.17 /cm.sup.3 is formed on a semi-insulating substrate, and a high purity buffer layer having a thickness of from about 0.2 to 5 .mu.m is usually inserted between the substrate and the active layer as described in Gallium Arsenide And Related Compounds (1976), Institute of Physics Conference Series, No. 33b, pp. 11-12. In order to prevent impurities in the substrate from exerting adverse influences on the active layer and to reduce a leakage current through the buffer layer, the buffer layer must be comprised of a high resistance crystal having a carrier concentration arising from residual impurities of not more than about 2.times.10.sup.14 /cm.sup.3 . Where the crystal for FET is allowed to grow by the MOCVD method, however, despite the crystal growth conditions so selected as to minimize the carrier concentration of the buffer layer, it has been difficult to decrease it below 2.times.10.sup.14 cm/.sup.3, which is considered to constitute a cause of inferiority in FET characteristics. This has placed a hindrance to industrial utilization of crystals obtained by the MOCVD method in FET for low noise amplifiers. Similarly, in the case of applying the crystals to high-output FET for power, FET for integrated circuits, etc., reduction in power efficiency and scatter of threshold voltage which are considered attributed to the shortage of buffer layer resistivity have been pointed out. In order to solve these problems, it has been attempted to add a dopant forming a deep level in the forbidden band of the growing buffer layer crystal to thereby reduce the residual carrier concentration by impurity compensation as taught in Journal of Crystal Growth, Vol. 44, pp. 29-36 (1978). However, satisfactory characteristics have not yet been attained due to influences from the deep level formed by the compensating dopant which is introduced into the buffer layer in a large quantity and due to transfer of the compensating dopant to an active layer which is subsequently formed on the buffer layer.
From the viewpoint of vapor-phase growth, a silicon impurity in the organic metal raw material is known as one of causes of the high residual carrier concentration in non-doped crystals formed by the MOCVD method and, hence, reduction of a silicon impurity in the raw material has been studied for improving crystal characteristics as described in Journal of Crystal Growth, Vol. 55, pp. 255-262 (1981). Nevertheless, even those crystals prepared from an organic metal whose impurity chiefly comprising silicon has been considerably reduced still suffer from scattering of crystal purity, proving unsuitable for stable use as crystals for FET.
In the light of the above-described circumstances, it has been demanded to produce a highly resistant crystal having a low residual carrier concentration without using a compensating dopant and to develop an FET using such a crystal as a buffer layer.
The inventors had previously conducted extensive studies to analyze causes why the epitaxial crystals prepared by vapor-phase growth using arsine do not have stable purity and are therefore unapplicable to devices requiring a high purity layer. As a result, it had been found that the donor impurity concentration in the epitaxial crystal is subject to great variation with lot-to-lot variation of the arsine source used as reported in The 46th Ohyo Butsuri Gakkai Yokoshu, 2a-E-3 (1985). According to the inventors, studies, when various crystal layers prepared using various arsine lots were applied to FET as a buffer layer on trial, crystals obtained from the most arsine lots turned out to be unsuitable for practical use. This means that, in using commercially available arsine, a deliberate choice should be made from various arsine lots before one can prepare a crystal having desired characteristics suited for use in FET with good reproducibility. Moreover, even if any chose may be determined, since arsine lots suitable for practical use are very few in number, it is difficult to produce a desired crystal on an industrial scale. Therefore, from the standpoint of industrialization of devices requiring high purity crystals, such as high-speed electronic devices including FET, it has been keenly demanded to stably supply arsine of improved purity and to develop a vapor-phase crystal growth method by the MOCVD method or hydride method using such high purity arsine.
Considering that the amount of impurities present in arsine is extremely small, stable supply of high purity arsine cannot be achieved unless one knows what and how much the impurities are with no analytical means available, what kinds of impurities are present, to what degree the impurities should be reduced, and how to purify the arsine source. However, it has been virtually impossible to solve these problems.